(1) Field of the Invention
This invention relates to a display source which is particularly, but not exclusively, suitable for use in a helmet mounted display or head mounted display.
(2) Description of Related Art
Traditional, as illustrated in FIG. 1, a prior art helmet mounted display 1 can be constructed to directly image a transmissive digital display device 2 using only primary relay optical elements 3a, 3b and 3c to transmit light generated by the digital display device 2 onto a visor 4, which is arranged to reflect light back to an exit pupil at the point of a viewer 5. It will be understood that the viewer 5 observes a forward scene through the visor 4 and the image produced by the digital display device 2 is superimposed upon the forward scene. The problem with this approach is that transmissive digital display devices 2 have an inherent drop in contrast related to the angle of incidence between the transmissive digital display device 2 and the primary relay optical elements 3a to 3c. Accordingly, light on axis to the transmissive digital display device 2, indicated by ray trace 6, provides good contrast at the point of the viewer 5. However, light off axis, indicated by ray traces 7 and 8, have a larger angle of incidence than light on axis ray trace 6 and therefore provide poorer contrast at the point of the viewer 5. Helmet mounted display primary relay optical elements 3a to 3c, by nature, provide a large numerical aperture and hence require a large angle of incidence to be present at the transmissive digital display device 2. This in turn leads to large variations in contrast for the head mounted display at the exit pupil at the point of the viewer 5.
FIG. 2, illustrates a way of improving the contrast for a helmet mounted display 10 according to the prior art using a reflective digital display device 11, a beam splitter 12 and an illumination source 13 rather than the transmissive digital display device 2 of FIG. 1. In operation, light from the illumination source 13 passes through illumination optic elements 14 and 15 prior to entering the beam splitter 12 for reflection within the beam splitter 12 to the reflective digital display device 11. Light incident on the reflective digital display device 11 can either be reflected so as to pass back through the beam splitter 12 to be directly imaged by primary relay optical elements 16A, 16B and 16C or to be prohibited from passing through the beam splitter 12. Light allowed to pass back through beam splitter 12 and hence the primary relay optical elements 16A to 16C, as indicated by on axis ray trace 17 and off axis ray traces 18 and 19, is incident on a visor which is arranged to reflect the light to an exit pupil for viewing by a viewer 21.
However, to use a reflective digital display device 11 in such a manner so to be directly imaged by the primary relay optical elements 16A to 16C requires a large beam splitter 12 and associated illumination optics 14 and 15 with a separate illumination source 13 to provide the required sized exit pupil to a viewer 21. This would require modification of the complex primary relay optical elements 16A to 16C to accommodate a large back focal length to allow room for the beam splitter 12, illumination source 13 and illumination optic elements 14 and 15.
Furthermore, directly imaging a digital display device 2 or 11 as described and illustrated either in FIG. 1 or 2, means that the primary relay optical elements 3a to 3c of FIG. 1 or 16a to 16c of FIG. 2 can only be designed for a specific digital display device 2 or 11. The primary relay optical elements 3a to 3c or 16a to 16c have a fixed focal length and any change in digital display device 2 or 11, for example size, will severely impact the optical performance of the helmet mounted display 1 or 10. That is, if the size of the digital display device 2 or 11 is decreased then the field of view presented at the exit pupil for a viewer 5 or 21 will be lost. Conversely, if the size of the digital display device 2 or 11 is increased, then display pixels around the periphery of the digital display device 2 or 11 will be wasted resulting in a lower resolution as viewed by a viewer 5 or 21 at the exit pupil. As is well known, the technology in digital display devices 2 or 11 is rapidly advancing and hence such digital display devices 2 or 11 quickly become redundant and superseded, thus it would be necessary to redesign the primary relay optical elements 3a to 3c or 16a to 16c of the helmet mounted display 1 or 10 each time a digital display device 2 or 11 became redundant.